This invention relates generally to pneumatic hammers and in particular it relates to a hammer of the type having a housing body defining an inlet air channel and an exhaust air channel, a hollow cylindrical member guided for a reciprocating axial movement within the body and a striking piston guided for a reciprocating axial movement within the cylindrical member.
In pneumatic hammers of this type, the arrangement of the reciprocating striking piston in a hollow cylindrical member which is guided for a reciprocating axial movement in the hammer housing brings about that vibrations resulting during the operation of the hammer are attenuated. The damping of vibrations of the hammer substantially improves the working conditions of the operator who no longer is exposed to excessive percussions which are hazardous to the operator's health.
The disadvantage of prior art structures of pneumatic hammers having means for damping vibrations is the fact that due to the axial freedom of movement of the hollow cylindrical member in the housing body, the hammer can be set into operation even if a work tool inserted into the cylindrical member is not in an effective contact with the working surface.
This undesired possibility results from the fact that upon actuation of the inlet valve, the pressure air starts flowing into the working hollow cylinder and sets the striking piston into motion. This condition, however, brings about a considerable chance of accidents inasmuch as the working tool which is insufficiently ensured against expulsion from the hammer can be pushed out and in addition during the idling operation of the hammer an unnecessary high wear of the pneumatic hammer takes place.
The above-described disadvantages of prior art structures of vibration attenuated pneumatic hammers have therefore the consequence that the attending personnel has to exert a high degree of precaution in order to prevent working accidents due to an unintended expulsion of the working tool from the housing body and to avoid the unnecessary mechanical wear of the hammer in the range of the mounting device for the insertable working tool.
In order to reduce the above-described disadvantages, safety means have been already devised in the vibration attenuated pneumatic hammers of this type. In such known safety devices, the axial displacement of the working hollow cylinder relative to the housing body of the hammer is utilized for controlling the inlet of the pressure air into the working hollow cylinder. For example, in the German utility model No. 72 11 456, a solution is known where entry of the pressure air into the interior of the working hollow cylinder when the latter is in its rest position is prevented by providing an air exit opening on a guiding pin on which the working hollow cylinder is axially movable relative to the housing body whereby in the rest position of the hollow cylinder, this exit opening is closed.
The disadvantage of this prior art solution for controlling the discharge of the pressure air into the working hollow cylinder is the fact that component parts for controlling the discharge of the pressure air are off-set from the center of the working cylinder and consequently are exposed to a relatively high wear. This wear in turn causes a relatively fast deterioration of the seal of the control channels and consequently the operational disturbances of the hammer itself. Moreover, in the rest position of the working hollow cylinder, it cannot be excluded that due to the deteriorated sealing surfaces of respective component parts of the hammer, pressure air in the form of a driving airstream at the open inlet valve and in the form of a leaking stream at the closed inlet valve, will enter the interior of the working hollow cylinder. As a result, the striking piston of the pneumatic hammer is set into operation in an uncontrollable manner and performs so-called unloaded blows. The striking energy of such unloaded blows and the tool mounted in the hammer housing encounters no resistance against a working surface and has a negative influence on the working life of the hammer. Apart from the previously discussed danger of expulsion of the insufficiently secured working tool in the hammer housing during the empty or unloaded blows and resulting danger of injury of bystanders, the arresting device for the working tool, the so-called tool lock becomes exposed to excessive blows of the free striking piston and may become so damaged that its function is impaired. In other words, apart from the disadvantageous effects relating to the insufficient operational safety, the occurrence of free-running or unloaded blows may cause also a momentary or permanent inoperativeness of the entire pneumatic hammer.